In Borneo, Stalagmites Tell Modern Story of Rainfall and Climate Change

On the tropical island of Borneo, some of the most spectacular caves in the world also hold a piece of an ongoing climate puzzle.

The caves of Gunung Mulu are a tourist destination and a UNESCO World Heritage Site, with large, open chambers and stunning formations. Those glittering stalagmites, which have been forming for tens of thousands of years, are what provided Georgia Institute of Technology doctoral student Stacy Carolin with a way to investigate how rainfall in this region shifted when past climate changed.

"Stalagmites are a very interesting proxy because they can be very well dated and they can also be measured at high resolution to give us oxygen measurements that we can relate to rainfall," Carolin said.

She analyzed samples that had broken off the formations to see what periods of time during a period called the last glacial period had high rainfall, and which had low rainfall. The last glacial period spanned from 110,000 to 10,000 years ago; during that time the Earth was colder and glaciers covered significantly more land.

What she hoped to discover was whether rainfall in the western tropical Pacific changed at the same time as some of the major climate shifts that scientists know happened during this period. The results of the research were published online yesterday in the journal Science.

Climate scientists find the last glacial period interesting because ice cores in Greenland and ocean sediment cores have shown that during this period there were sharp shifts in global temperatures. These could provide an analogy for what might happen now as our world warms.

A look at a distant mirror?
There were two sets of climate shifts from the last glacial age that Carolin was hoping to correlate with precipitation changes in Borneo.

One set is called Heinrich events, which were periods when large numbers of icebergs broke off glaciers and traveled around the north Atlantic Ocean. The fresh water added to the ocean by these melting icebergs may have changed ocean circulation and climate.

The other set is a series of rapid increases in the Earth's temperature known as Dansgaard-Oeschger events.

Evidence for these rapid warming events, on the order of 10 degrees Celsius in just 30 or so years, has shown up in the Greenland ice core, said Kim Cobb, the paper's second author. Cobb, a climate scientist at Georgia Tech, is Carolin's Ph.D. adviser, and the stalagmite samples were analyzed at her laboratory.

As Cobb explained, the tropical Pacific, through phenomena like El Niño, plays a very large role in precipitation and global weather patterns like monsoons today. Because of this, scientists believe it is important to see how historical climate changes documented in one part of the globe, like the north Atlantic, play out in that region.

"We were trying to understand what effect they had in the deep tropical Pacific," she said.

As Cobb explained, climate scientists still lack a good understanding of how climate change will alter precipitation patterns. "That's something that lies at the core of food production, resource management -- kind of the core survival aspects of our society and infrastructure that we have built," she said.

Other research on stalagmites in China has shown that the East Asian monsoon changed at the same time as the Heinrich and Dansgaard-Oeschger climate changes. It wouldn't be that much of a jump to think this also happened in areas closer to the equator.

But to her surprise, Carolin found that the changes in precipitation recorded in the Borneo stalagmites did not correspond nearly as well with one set of climate changes in particular, the Dansgaard-Oeschger events.

Matching the models with stalagmites
There was a much clearer correlation between less rainfall in the region and the Heinrich events, Carolin said.

"What we saw was something that looked very different. It wasn't just a muted version of the Chinese stalagmites," Cobb added.

Gavin Schmidt, a climate scientist and modeler at the NASA Goddard Institute for Space Studies, said this sort of research is useful for modelers, who can take these results and see whether they show up when they run their models.

If they do, researchers know the models are accurately replicating how climatic shifts in one region of the Earth affect other parts of the globe.

"They're putting out stuff that says, OK, look, these are really good targets for you to test your models against," he said. "Somebody like me sees something like this and we say, oh, I wonder if we did that [in a model], how well would we do?"

As it turns out, Schmidt is already halfway there. In 2011, he co-authored a paper in the journal Earth and Planetary Science Letters in which he modeled how the Heinrich events might have affected stalagmite formation in the tropical western Pacific.

Over the phone, he eyeballed his results with what Carolin and Cobb came up with in their recent paper.

"Now, the moment of truth," he said, looking at his figure.

What he saw was impressive. The results from the Carolin paper, with real data from real stalagmites, are almost "exactly what the model predicted in 2011," he said.